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Calvin-Benson cycle
The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of thethylakoid membranes. These reactions take the products(ATP and NADPH)of light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration. Despite its name, this process occurs only when light is available. Plants do not carry out the Calvin cycle by night. They, instead, release sucrose into the phloem from their starch reserves. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean Acid Metabolism); CAM plants store malic acid in their vacuoles every night and release it by day in order to make this process work. Tossup Questions # Pettersson and Ryde‐Pettersson created a model of this process that hinted at a second steady state condition in it. One intermediate in this process is stabilized by lysine 334. That lysine is found in an enzyme whose states in this process are referred to as E, EC, and ECM. That enzyme requires a namesake activase enzyme for proper functioning. In this process, aldolase creates 2 SBP from (*) 2 DHAP and 2 E4P. Its key protein requires magnesium ions and this process regenerates RuBP using 5 of the 6 G3P produced in another step. Oxygen competes with carbon dioxide in the main enzyme for it during photorespiration. For 10 points, name this cycle catalyzed by RuBisCO that fixes carbon into sugars using ATP formed during the light dependent reactions, a part of photosynthesis. # Its last two phases are reduction and regeneration. Important enzymes in these steps include aldolase, transketolase, and sedoheptulose bisphosphatase. C4 plants use PEP carboxylase to form oxaloacetate before beginning these reactions, allowing them to increase productivity by carboxylating more often than can C3 plants. NADPH is oxidized when it donates an electron to form G3P, which can be converted to glucose. However, five out of every six G3Ps are used to regenerate RuBP, which is attached to CO2 by Rubisco in the first step. For 10 points, name this reaction cycle that occurs in the stroma and produces sugars from carbon dioxide. # Enzymes in this pathway can be activated in the presence of reduced thioredoxin (THY-oh-ree-DOX-in). One enzyme in this pathway, glyceraldehyde-3-phosphate dehydrogenase (GLISS-ur-AL-duh-hide THREE FOS-fate dee-hie-DRAW-jen-ase), uses (*) NADPH as an electron donor. In addition to six molecules of G3P, this process also produces one carbon available to be converted into a sugar. Rubisco is a catalyst in, for 10 points, what cycle of reactions that takes place in chloroplasts and are part of the dark reactions? # The reduction of thioredoxin by ferrodoxin activates many enzymes in this process, while another enzyme central to it is activated by the binding of magnesium ions to a carbamate group. It also sees the epimerization of Xylulose-5-phosphate to Ru5P. Its second stage sees phosphorylation of 3-PGA to yield six glyceraldehyde-3-phosphate molecules, only one of which is later used to produce glucose. For 10 points, name this carbon-fixing pathway which occurs in the stroma of chloroplasts, a cycle sometimes called the "dark" reactions. # It was elucidated at Berkeley by exposing Chlorella pyrenoidosa to a radioactive substance and killing its cells with boiling methanol. Noted for using sedheptulose-1,7-bisphosphate phosphatase, this process uses five molecules of glyceraldehyde 3-phosphate to regenerate the starting material. This process occurs in the stroma of chloroplasts, and is associated with the most abundant protein on Earth, rubisco. For 10 points, name this eponymous cycle that is part of the light independent reactions of photosynthesis.